In development of non-volatile memories and DRAMS for integrated circuits, memory storage capacity of each generation of DRAM increases, and the area of individual memory cells decreases. To maintain sufficient capacitance in each cell for charge storage, deep trench isolated electrode may be used to increase the capacitor plate area. On the other hand, since the dielectric constant of conventional capacitor dielectrics, typically silicon dioxide is low, (about 4), interest has turned to dielectrics having higher dielectric constants which include ferroelectric materials such as ferroelectric perovskites of the general formula ABO.sub.3, e.g. lead zirconium titanate, strontium titanate, barium titanate, etc.
However, problems of poor adhesion or unwanted interfacial diffusion of impurities may occur when incorporating materials such as lead titanate zirconate (PZT) thin films into conventional capacitor structures in bipolar and CMOS processes for fabrication of integrated circuit devices.
The selection of appropriate conductive electrode materials and barrier layers between the electrode material and ferroelectric capacitor dielectric is a critical factor in fabrication of ferroelectric memory cells to avoid undesirable interfacial diffusion and reactions.
Noble metals have been used as capacitor electrodes, for example Pt has been used as a bottom electrode. However a major drawback is the rather high temperature of 700.degree. C. during processing which is required to initialize the formation of a ferroelectric perovskite phase on a platinum electrode. Other electrode materials such as Au or Ni alloys result in formation of Au.sub.2 O.sub.3 or NiO insulating layers at the electrode/ferroelectric interface, and are thus unsuitable. Al or Al alloys cannot be used due to the problem of oxide formation at the interface, and also because of their low melting temperature they are incompatible with processing temperatures above 700.degree. C. required for crystallization of the ferroelectric perovskite. Indium tin oxide has been used, but this material initiates the formation of a PbO rich phase at the interface with PZT. Electrodes such as W, Ti, or doped Si result in poor adhesion of the PZT layer upon crystallization. While stainless steel has been used as a substrate, it is unsuitable for VLSI applications.
Various metals and conducting metal oxides, e.g. Ru and their oxides have been proposed as barrier layers between a more conventional electrode material, e.g. polysilicon, and the ferroelectric PZT layer to prevent undesirable interfacial diffusion and reactions.
Furthermore, a metal oxide layer, e.g. cuprate oxide or bismuthal oxide having a perovskite structure has been found to aid crystal growth thereon of a perovskite ferroelectric, as described in U.S. Pat. No. 5,155,658 to Inam issued Oct. 13, 1992, entitled "Crystallographically Aligned Ferroelectric Films Usable In Memories and Method of Crystallographically Aligning Perovskite Films".
As described in U.S. Pat. No. 5,003,428 to Shepherd entitled "Electrodes For Ceramic Oxide Capacitors", ruthenium oxide is a conductive electrode material which is suitable for use in ferroelectric capacitors. Ruthenium oxide is an oxide having tetragonal rutile structure with unit cell dimension of a=4.49.ANG. and c=3.11.ANG., which would be expected to initiate the crystallization of a perovskite ferroelectric phase of PZT at lower temperatures than normally observed on materials lacking a tetragonal structure, i.e. W, Pt, Al and Au have cubic structures.
Further, PZT and RuO.sub.2 have been shown to display negligible interdiffusion at temperatures less than 500.degree. C.
Various conventional methods are known for forming RUO.sub.2. RUO.sub.2 may be prepared by oxidation of Ru metal at 1250.degree. C., oxidation of RuCl.sub.3 at 900.degree. C. and chemical vapour deposition (CVD) from volatile oxides at 1090.degree. C. Lower temperature methods of RuO.sub.2 film formation include the deposition of a ruthenium trichloride titanium tetra-n-butoxide solution onto a Ti substrate followed by heating. It may also be formed by the deposition of a ruthenium trichloride/ammonium molybdate solution onto a SnO.sub.2 substrate followed by heating at 400.degree. C. Films have also been prepared recently by sputtering techniques as noted in the above mentioned U.S. Pat. No. 5,003,428 to Shepherd.
In a publication in Japanese Journal of Applied Physics, Vol. 31, page 135, January 1992, there is mentioned a metallo-organic deposition from ruthenium octyrate in 4-methyl-2-pentanon at 600.degree. C.
For use of RuO.sub.2 in integrated circuits, deposition temperatures must preferably be below 700.degree. C. to avoid thermal damage to integrated circuit elements. Also, because chloride ions may have a deleterious effect on Si based integrated circuits, a chloride free process is desirable.